Hematopoietic stem cells (HSCs) give rise to all blood cells and are able to both self-renew and differentiate into multilineage progenitors. The rarity of the hematopoietic stem cell, coupled with an inability to maintain these cells in culture for a significant period, has greatly hindered the biochemical and molecular characterization of these cells. It is reasonable to postulate that expression of specific genes regulates hematopoietic stem cell functions. Thus, insight into HSC functions can be gained by the identification of genes that are expressed in a population of HSCs that has extensive self-renewal capability. We used array analysis to identify genes expressed by a highly enriched population of self-renewing HSCs. The result of this array has demonstrated that there is a class of genes that was highly expressed in HSCs as well as a single mature lineage. To begin to understand the biological functions of genes with this type of expression pattern, we have begun to study one clone, designated RGS18, that was highly expressed in HSCs and myelomonocytic cells. RGSI8 showed significant homology to RGS (Regulator of G-Protein Signaling). RGS proteins function as a molecular off switch for trimeric G-protein signaling. The RGS proteins have been implicated in lymphocyte function by modulating signaling through 7-transmembrane receptors, in chemokine signaling and homing, and in differentiation and transformation. Over-expression of RGS 18 in myelomonocytic U937 cells induces differentiation and cell death, and over-expression in pro-B cell line BaIF3 inhibits cell migration mediated by chemokine SDF-1alpha In this proposal, we attempt to identify and characterize the biochemical functions and in vivo role of RGS 18 in mouse hematopoiesis. Specifically we plan: Specific Aim # 1. To determine the biological function(s) of RGS18 in hematopoiesis. Specific Aim # 2. To determine the mechanism by which interferon-gamma regulates RGS 18. Specific Aim # 3. To study biochemical functions of RGS18.